主、备切换；网络技术；继电器 Abstract: Traditionally, the switch of PLC between master and slave always uses relay, it is reliable and secure, however, slave PLC can not acquire status variables with this method, such as analogue values, digital input status, and so on; if there were some sequence flags or must keep relays of peripheral equipment, program can not run as usual, and peripheral equipment may be stopped when the switch completed. This paper provides a set of method, which is used in GE Fanuc Series 90-30 PLC with network technology; it can solve problems produced by the switch, and acquires good result in real project. Keyword: switch between master and slave; network technology; relay
